Javascript target prob

I have a page similar to what I'm typing in now, I want to be able to add 'Formatted' Text to my page..

I'm using a Java script that adds the tags to a prompt box, its works fine how ever it submits it to a new page rather than add it too my text field which is called 'Page1'..   any ideas?

The script which i'm putting in the /HEAD

<SCRIPT language="JavaScript">

function doPrompt()
{
 var yourName
 yourName = window. prompt("Please enter text to be Bold formatted.")
 document.write("<b>" + yourName + "</b>")
}
</SCRIPT>

The hyperlink.

<a href="#" onclick="doPrompt();">Bold</a>

Many thanks in advance..